V. Study Protocol A. Data Collection: Subjects will be brought to the MGH Clinical Research Center for an initial screening visit in which eligibility criteria will be reviewed (with exclusion/inclusion as applicable), and they will be given an ECG recording. Eligible individuals for the study visits will be brought back later to the MGH Clinical Research Center on two separate days separated by at least one week (to ensure washout). On each visit, they will receive study drug (either 400 mg oral moxifloxacin or placebo) allocated by double blinded block randomization (by sex and genotype group separately) at 10AM in the fasting state (statistician will hold the randomization key; all other study personnel will be blinded until study closure). Women will be screened for pregnancy using a urine HCG at the time of both study visits. They will undergo 6 ECGs at time 0 and at 30 min intervals thereafter, in the seated position after at least 15 min rest using fixed electrode placement for a total of 6 hours following administration of study drug. At 2, 4 and 6 hours we will draw a blood sample for plasma moxifloxacin determination by mass spectrometry. All ECGs are uploaded to a research partition of the GE MUSE 7 ECG database of the Massachusetts General Hospital, and the 12 SL algorithm applied for interval measurement, as used in prior studies by our group. We will monitor the QT every 30 minutes throughout the protocol and will identify any subjects with marked QT prolongation. Any subject who has a QTc >500 ms will be kept in the HCRC for further observation, with the decision to either follow-up at the HCRC every morning until it is less than 500ms or be admitted to the hospital for telemetry based on the degree of prolongation and clinician's judgment.